A circuit for measuring respiratory waveform with impedance method is a small signal detection circuit, which is widely applied to patient monitor. Human thorax has certain basic impedance itself, and the impedance values may be slightly changed in a range from 0.1 ohm to several ohms under the frequency range from about 0.1 Hz to 2 Hz with the variation of thorax volume caused by respiratory exercises. In a method for measuring respiration with impedance methods, a stimulating signal, of which frequency is from 20 kHz to 100 kHz, is applied to a human body through electro cardiac leads, then said signal is modulated with the variation of thorax volume and the envelope of the modulated wave is deemed as a respiratory waveform. Such detection method by means of modulation can eliminate the influences resulting from contact impedance of electrodes, low frequency drift of circuits and low frequency noise.
The EFT interference (i.e., electrical fast transient interference) in power supply is caused by switching inductive loads in power grid. In different circumstances, the amplitude of the EFT interference may be up to 500-4000V with its rise time of nanosecond order, wherein a single pulse may typically persist for several ten nanoseconds, and there are generally several ten pulses with the persistence time ranging from nought point several milliseconds to several milliseconds per burst. EFT interfering signal tends to be overlapped with stimulating signal for respiratory detection through distributed capacitance due to the characteristic of high frequency and high voltage. The EFT interfering signal is returned to the ground via distributed capacitance, human body and electro cardiac leads so that the modulation circuit at the preceding stage of respiratory detection circuit may be directly affected.
Since the basic impedance of respiration generally ranges around 200-2000Ω and there is 100Ω of current-limiting resistance for defibrillation in some electro cardiac cables, the basic impedance may be up to around 4000Ω and the variable resistance lower to around 0.2Ω. Therefore, the amplification factor of measurement circuit is highly required, even up to several ten thousands times, and the occurrence of the slight interference at the preceding stage can bring great effect on respiratory signal. High frequency interference pulses such as EFT, etc. may interfere with the respiratory detection circuit with space coupling or inside the patient monitor, and impose a transient high voltage shock upon the circuit, instead of stable existence all the time, so that it is difficult to eliminate these pulses from respiratory detection signal by means of low-pass filtering. A fundamental solution is to switch off the current loop of interfering signal. For example, by means of adjusting the mechanical structure of the entire machine, the distributed capacitance between power supply of patient monitor and floating circuit is reduced so far as possible. In practice, the power line is set to keep a certain distance as long as possible from the electro cardiac leads so as to reduce the direct coupling, etc.
Below are two methods used for suppressing EFT interference in prior art.
I. The interference may be suppressed by increasing signal-to-noise ratio by means of raising the amplitude of stimulating signal and reducing the amplification factor of the output from the modulation circuit. The effect of such method is limited due to the following two aspects, one is that the amplitude of stimulating signal can not be increased unlimitedly because of the restrictions of power range of the amplifier circuit and safe current via human body, and the other is that the amplification factor at subsequent stage can not be lowered too much in order to ensure the detecting sensibility of respiratory waveform.
II. The performance of resisting EFT interference may be improved by increasing the degree of impedance match at the input sides of leads (such as RA, RLL) for collecting respiratory signal, because if the impedances at the input sides of the leads do not match, a common mode signal which is the EFT interfering signal coupling to the leads will be converted to a differential mode signal along with the result of the occurrence of interference. However, such method requires high precisions of elements and PCB wiring, so it is hard to obtain desired effect in practice.
Thus, from the above description, it can be concluded that the prior art is disadvantageous in limited effect of resisting EFT interference and high demand on circuit design.